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PRINCIPLES OF EMBRYOLOGY 



to become mesoderm and nothing else; and it is then, at this very early 

 stage, that the immunological techniques show that myosin is already 

 present, hi the next stage, of the long primitive streak, Ebert tested not 

 only whole blastoderms but parts of them, as showoi in Fig. 10.17. As far 

 as the tests went, myosin was present in all the regions which contain 

 presumptive mesoderm. By this stage, the individuation of the mesoderm 



Figure 10.17 



Parts of the chick blastoderm tested immunologically for their content of 

 cardiac myosin. The intensity of their reaction with test sera is indicated by 

 the density of dotting. Above, streak and head process stages cut into three 

 strips. Below, the central strip cut into a middle and two lateral portions. 

 (After data of Ebert 1953^.) 



is beginning, and different parts of it are acquiring more specific properties 

 (p. 187). By the head process stage, this regionalisation has gone some 

 way; and, further, by this time the presumptive heart mesoderm will 

 have been completely invaginated and moved out laterally to either side 

 of the streak. Ebert finds, in accordance with this, that the myosin reaction 

 has become very weak both in the posterior part of the blastoderm 

 (posterior to the presumptive heart) and in the anterior part of the streak 

 (between the two presumptive heart rudiments), and has disappeared 



